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Microsoft has defended its decision to release a Windows-only security patch for its Office program after a researcher warned it put Mac users of the software at risk.

Swa Frantzen, in a blog item posted to the SANS Institute's Daily Handler's Diary, said a bulletin Microsoft issued Tuesday violated the company's own position on "responsible disclosure," which admonishes security researchers to publicly divulge vulnerabilities only after a software maker has had time to fix them. What's more, he said the move would make it easier to attack Office for the Mac.

"We all know from past experience the reverse engineering of patches back into exploits starts at the time - if not before - the patches are released," Frantzen wrote. "Typically, it takes between hours and a few days or so to complete this if it's easy to exploit." He went on to note that even Microsoft's own bulletin warned that the flaws were easy to target.

The brouhaha erupted one day after Microsoft issued a patch fixing 14 vulnerabilities in PowerPoint, including one that Microsoft warned last month was being exploiting in "limited and targeted attacks" in the wild. Christopher Budd, Microsoft's lead security response spokesman, said the decision came only after a great deal of deliberation. Ultimately, he said, releasing the Windows updates ahead of those for the Mac ensured the greatest number of customers were protected.

"We've not faced this particular situation before, but after a lot of discussion what we decided to do is go ahead and provide updates for 100 percent of the customers on products that we have observed under these limited and targeted attacks while continuing the work on these other products," Budd told El Reg. "Ultimately, the deciding factor was what do we believe most effectively protects the customers."

Budd said Microsoft has yet to see attacks targeting the same PowerPoint flaws in Mac versions of Office, and he disputed Frantzen's contention that the Windows patches could easily be reverse engineered to write Mac-specific exploits. After realizing the PowerPoint patches were complete for Windows but not for the Mac, Microsoft took this information into consideration in deciding to release what it had rather than wait until it had all versions covered.

In addition to withholding a fix for Mac versions, Microsoft has also yet to deliver a patch for the PowerPoint version of its more slimmed-down suite called Works. While all the programs share certain vulnerable code, the exploits Microsoft researchers have seen to date work only on Windows versions, Budd said.

"To take an attack against Office of Windows and make it viable against those other two products requires a degree of retooling that frankly we don't see in the security research space right now," he said. ®